NEW YORK (CNN/Money) - Recently in New York, there was a sight so jarring even the city's jaded commuters took notice: a throng of scantily clad men and women in front of Grand Central Terminal, flashing underwear with "Booty Call" emblazoned on the backside.

Pedestrians and car drivers whipped out digital cameras. Workers in nearby highrises looked on from office windows. Photos posted on the Internet soon drew viewers from all over the world.

The skivvy sighting wasn't about self-expression. It was part of an orchestrated campaign by New York Health and Racquet Club to promote a butt-building class for J. Lo wannabes.

"It was fantastic," said J. Travis, brand manager for New York Health and Racquet Club.

Like a lot of companies, the regional sports club has discovered there's a cheaper and perhaps more effective way to reach consumers than the typical television ad or radio spot.

Called "guerrilla marketing," the idea is to catch increasingly finicky and jaded consumers unaware through highly unconventional -- and sometimes shocking -- means, including blanketing city landmarks with stickers of a company logo or using graffiti to paint a brand name.

Other irreverent and insidious forms include "dog-vertising" and "bra-vertising."

Marketers say these campaigns are relatively cheap to pull off, starting at $10,000 for a modest, one-city campaign to the low six-figures for a multi-city initiative -- well below the cost of a 30-second slot on primetime television.

The campaigns may also be more effective. Consumers are bombarded with roughly 3,000 product messages a day, according to Brad Adgate, senior vice president of corporate research at New York branding firm Horizon Media. "The strategy is to stand above the noise," he said.

Guerrilla tactics don't get the millions of viewers that television broadcasters can deliver, but not many 30-second spots deliver the buzz of a successful ambush.

A Web site created especially for the New York Health and Racquet Club's 'booty call' campaign featured photos that effectively re-broadcast the message at no extra cost. Some 790,000 viewers have visited www.ass-vertise.com, said Darren Paul, managing director at Night Agency, the marketing shop that oversaw the "Booty Call" stunt.

"It would have cost us half a million dollars to get that kind of publicity," said Travis.

Hold that tiger

The idea is catching on.

Le Tigre, the apparel manufacturer whose polo shirts with a sprinting tiger logo were hip in the 1980s, is embracing guerrilla marketing to mount a comeback. In June workers took to the dirty streets of 5 major cities and using cleaning solvent formed the company logo from the grime. At the U.S. Open tennis tournament next month, shirtless men and bikini-clad women will hand out free t-shirts and wrist bands with the Le Tigre logo.

Minnesota fishes for tourists in Chicago.

Another off-beat campaign was launched this spring by Minnesota state officials looking to boost tourism to a local resort town. On some of Chicago's busiest streets, hired workers wearing life vests and flannel sat in canoes and promoted the virtues of Ely, Minnesota. They pretended to fish and even tried to row down the cement. (They didn't get very far.)

Even traditionally staid companies are experimenting. Citigroup, the financial services giant, is looking to grow its money management services for high-income female workers with a street campaign next month in New York, Los Angeles and Chicago. Hired helpers, dressed in green trench coats, will seek out passers-by who seem to fit the company's target demographic and offer them free coin purses. Inside will be a coin and a pitch telling recipients that, without an investment adviser, they're giving away money just like Citigroup.

Crossing the line

Guerrilla marketing can back-fire. Cities don't always welcome the intrusion on public space. And marketers warn of a consumer backlash from stunts that go too far.

"You have to be careful and understand the rules of the game," said Paul. "The field of guerrilla marketing is similar to advertising. There's good and bad marketing. There's good and bad execution. And there are negative case studies of companies crossing the line."

In 2001, after IBM spray-painted "Peace, Love and Linux" on San Francisco streets, city leaders levied a $120,000 penalty against the company.

More symbolically, in 2002 New York City threatened to sue Microsoft after it blanketed Manhattan with thousands of butterfly stickers that weren't easily removed. The software giant wound up apologizing and assisted with the cleanup. The city didn't take the company to court, but it did fine Microsoft $50 for littering.

More recently, video game maker Acclaim Entertainment announced it would promote an upcoming release in the United Kingdom with ads at bus shelters that seeped fake blood onto the sidewalk. The company never went through with it. Marketing experts agreed it would have been a risky idea.

Even so, when it comes to ambush marketing, the old adage that there's no such thing as bad publicity applies.

"Did they not work?" asked Paul, referring to some of the more controversial campaigns. "Well, we're still speaking about them."